1. Field of the Invention
The present invention relates to fuel cell devices, and in particularly to solid oxide fuel cell (SOFC) devices having a gradient interconnect therein.
2. Description of the Related Art
Fuel cells such as proton exchange membrane fuel cells (PEMFCs) or direct methanol fuel cell (DMFC) operating at a low temperature and fuel cells such as molten carbonates fuel cells (MCFC) and solid oxide fuel cells (SOFCs) operating at a high temperature have already been developed. Advantages of SOFCs are low pollutants, high energy conversion efficiency, and no electrolyte evaporation, leakage and corrosion since the electrolyte therein is formed in a solid state. Thus, SOFCs have long operating lifespan.
The SOFC comprises an anode, a cathode, a solid oxide electrolyte, and an interconnect plate (also called bipolar plates or interconnects). The interconnect plate is a key component of the SOFC and materials thereof can be either made of ceramic materials or metal materials. The interconnect plate junctions a cathode and an anode of two adjacent single cell units and functions as a physical barrier for protecting materials in the cathode electrode from the reduction environment of the cathode end of the SOFC. Additionally, the interconnect plate protects the anode electrode materials from the oxidized environment of the anode end of the SOFC.
Metal interconnect plates made of materials such as iron-based materials are typically employed in SOFCs. Ceramic oxides such as LaCrO3 is a material used in interconnect plates, which allows high operating temperature. However, LaCrO3 is difficult to process due to poor ductility when compared to iron-based materials and is expensive.
Therefore, iron-based materials are mainly employed in SOFCs. However, iron-based materials, used with solid oxide electrolyte of yttria-stabilized zirconia (YSZ) have a junction issue at a ceramic/iron interface therebetween. Mixing of hydrogen and oxygen may occur at an edge of the solid oxide electrolyte, thereby negatively affecting sealing of the SOFC. In addition, the iron-based materials will be oxidized at higher temperature in air.
Moreover, since the high operation temperature and thermal cycling performed during the operations of the SOFC, high thermal stresses are thus generated. Junctions between components such as the solid oxide electrolyte, the electrode and the interconnect plate of the SOFCs which are composed of various materials may be thereby affected by the thermal stresses formed during high temperature operating and thermal cycling operation thereof. Therefore, cracking of components in the SOFC may thus happen and thereby damage the mechanical structure and integrity of the SOFC. Leakage of the reaction gas of the SOFC is thus happened.
Therefore, there's a need to improve the structure of the SOFC to solve the sealing issue, the oxidation issue at high operating temperatures and mechanical structure integration issues caused by using the conventional solid oxide electrolyte and interconnect plate.